Genetics Spring 2015

Chromosomes and Sex-Chromosomes Inheritance I

Outline

Sex-Chromosome Inheritance – Sex Determination

•Females are homogametic (X); males are heterogametic (X or Y).•Autosomes are all the other chromosomes.•X an Y associate during meiosis, at a small region of homology.

•Reciprocal crosses for X-

linked eye color gene (w+

or w allele) give different results

(not the same progeny).

•Males therefore

exhibit recessive traits

carried on the X ch:

X-linked traits

Sex-Chromosome Inheritance – X-linked Inheritance in Drosophila (reciprocal crosses)

To test the role of

parental sex chromosomes on a given inheritance. Is the trait sex linked

or autosomal? (sex linkage)

•Seen in homozygous dominant and heterozygous in female and hemizygous dominant in males.•X-linked dominant traits occur in both sexes, but affect more females than males. •X-linked dominant traits do not skip a generation, as the mother potentially passes the trait on to all her children (sons and daughters equally affected).•X-linked dominant traits do not skip a generation, as the father passes the trait on to all his daughters but not to his sons (as the Y needs to be passed on).

Human Pedigree Analysis –X-linked Dominant Inheritance

Write a genotype underneath each individual using X, Y, A and a as symbols.

Pedigree showing full penetrance

Rett syndrome (dominant) is a progressive neurodevelopmental disorder almost exclusively affecting females.

Find characteristic

s

Human Pedigree Analysis –X-linked Recessive Inheritance•Seen in homozygous recessive in female and hemizygous recessive in males.•X-linked recessive traits occur in both sexes, but affect more males than females.•X-linked recessive traits tend to skip a generation.•X-linked recessive traits are not passed on from father to son, as the son inherits the Y-chromosome from his father and not the X-chromosome (affected males receive allele from mother).

Write a genotype underneath each individual using X, Y, A and a as symbols.

Hemophilia (recessive) transmission among Queen Victoria's descendants

Find characteristic

s

Human Pedigree Analysis –Y-linked Inheritance•Seen in males only as and traits are passed on from father to sons.•Y-linked traits are very rare as the Y chromosome is small and does not contain many genes.

Write a genotype underneath each individual using X, Y, A and a as symbols.

Hypertrichosis of the ears

Find characteristic

s

Little round shapes denote miscarriages(Macias-Flores et al., 1984)

Not all hypertrichosis cases are Y-linked …….in this case, hypertrichosis is ……. Check a few characteristics of all the inheritance patterns and make a diagnosis.•Xxxxx

•Xxxxx•XxxxxMy diagnosis is:

Not all hypertrichosis cases are Y-linked and XXXXXX (your previous diagnosis) …….in this case, hypertrichosis is ……. Check a few characteristics of all the inheritance patterns and make a diagnosis.

•Xxxxx•Xxxxx•XxxxxMy diagnosis is:

(2006) http://abcnews.go.com/Primetime/Video/playerIndex?id=2264698